Add optional rank in <product>

Author: tomschr

src/docbuild/config/xml/data/product-config-schema.rnc

@@ -445,6 +445,13 @@ div {
     ]
     attribute family { xsd:IDREF }
 
+  ds.rank.attr =
+    ## Rank of this product in the homepage listing (lower ranks are listed first; products with the same rank are sorted alphabetically by product name)
+    [
+      db:refpurpose [ "Rank of this product in the homepage listing (lower ranks are listed first; products with the same rank are sorted alphabetically by product name)" ]
+    ]
+    attribute rank { xsd:PositiveInteger }
+
   ds.docset-sort.attr =
     ## Whether to sort docsets of this product Z-A9-0 (`descending`, default) or 0-9A-Z (`ascending`)
     [
@@ -473,6 +480,7 @@ ds.product =
       ds.site-section.attr? & # TODO: Do we still need this attribute?
       ds.series.attr &
       ds.family.attr &
+      ds.rank.attr? &
       ds.docset-sort.attr? &
       ds.gated.attr?
     ),